Spectroscopy of molecules with unstable nuclei

With electronic, vibrational, and rotational energy states to account for, conducting spectroscopy of molecules can get complicated (see, for example, Physics Today, October 2019, page 18 ). The task becomes even trickier when components of the molecules are decaying during the analysis. Now, for the first time, researchers have obtained laser spectroscopic measurements of a radioactive molecule with a half-life short enough to be relevant to experiment. Led by Ronald Fernando Garcia Ruiz of MIT and Robert Berger of the Philipps University of Marburg in Germany, the research team used the ISOLDE facility at CERN (shown above) to probe radium monofluoride and determine some of its electronic energy transitions, which had been predicted in quantum chemistry calculations. The proof-of-principle demonstration is an initial step toward searching for symmetry violation through precision spectroscopy of such exotic molecules.

Garcia Ruiz, Berger, and colleagues produced on the order of a million radium nuclei per second by firing protons at a uranium carbide target; some of the newly formed Ra then reacted with tetrafluoromethane gas to form RaF⁺ ions, which were separated and neutralized to form molecular RaF. Next, the researchers exposed the molecules to laser pulses designed to home in on their excitation energies. To quickly cover a wide swath of frequencies in the visible and IR, the team used three broadband lasers that probed the molecules, reflected off a mirror, and then probed the molecules again from the opposite direction; the reflected pulse explored a different frequency range due to a Doppler shift in the frame of the speedy molecules. A higher-power laser ionized any excited RaF molecules into RaF⁺, which were deflected via electric field onto a particle detector. The technique yielded the energies of multiple ground-to-excited RaF transitions.

Although the radium isotopes in the study (mass numbers 223–26 and 228) have half-lives on the order of days or years, Garcia Ruiz, Berger, and colleagues say the technique should be effective for analyzing species with half-lives of less than a second. Next, the team plans to use narrower-band laser pulses to obtain more precise spectroscopic measurements of RaF. To translate those measurements into stringent tests of fundamental physics (see the article by Dave DeMille, Physics Today, December 2015, page 34 ), researchers will have to overcome signal-to-noise limitations associated with the low number of particles under study. Still, radioactive molecules have the potential to reveal anomalies that stable ones cannot: For example, the ²²⁴Ra nucleus has been shown to be pear-shaped , an asymmetry that should greatly amplify the signal of the nuclear electric dipole moment, a property that should be highly sensitive to the influence of particles beyond the standard model.

Spectroscopy of molecules with unstable nuclei could also prove useful for fields like astrochemistry. In 2018 researchers observed the spectral signature of radioactive ²⁶AlF in the emissions of a nova . With the new technique, a team could presumably pin down the spectra of ²⁶AlF and other molecules in the lab to help identify and confirm their presence in space. (R. F. Garcia Ruiz et al., Nature 581, 396, 2020 .)